Grey Reef sharks are often a familiar sight to divers as they are inquisitive and known to approach. They are one of the most commonly sighted sharks in the Red Sea and Indo-pacific, often seen at popular dive sites like St.John's Reefs in the southern Red Sea, and in relatively shallow coral reefs near natural bays and drop offs.
This species has a streamlined body with a long, broad but rounded snout and large, round eyes. They can be identified by a black edge margin on their caudal (tail) fin, plain or small white-tipped medium sized dorsal fin and dark tips on other fins. They have narrow pectoral fins and do not have a dorsal ridge. Most grey reef sharks are smaller than 1.9m in length, and they are usually found in depths less than 60m, but can dive to depths of 1000m. They are obligate ram ventilators, that means that they need to generate a constant waterflow over their gills to be able to breath.
The Grey Reef Shark - Carcharhinus amblyrhynchos
Typically, Grey reef shark colouration is light to dark grey above, with paler to white undersides. Many sharks exhibit this countershading as it enables them to be hidden both from above and below. Grey reef sharks also get sun tans! Individuals that spend more of their time nearer the surface tend to be darker grey, as the suns rays darken their skin.
Grey reef sharks are fast, agile, predators, primarily feeding on bony fishes, cephalopods and crustaceans, with larger individuals usually eating a greater proportion of cephalopods (primarily squid and octopus).
They are most active at night, spending the day on the reef in groups of up to 20 and splitting up in the evening to hunt in more open waters. Whereby they sometimes also hunt in groups because social foraging increases the capture efficiency as the success is greater than for solitary foraging. They especially excel at capturing fast-swimming fish in open water, which complements feeding alongside whitetip reef sharks which have a preference for feeding in reef caves and crevices. Heterospecific foraging has also been documented, where whitetip reef sharks attract grey reef sharks as these cause disturbed fishes to leave their protection of the reef. This opportunistic relationship allows the grey reef sharks access to prey that otherwise would not be available to them. The benefits of this relation are generally one way as only rarely whitetip sharks benefit, but it drastically increased the predation success grey reef sharks. Unlike this relationship, it is thought that they compete with sandbar sharks as they have very similar feeding habits, and in areas with high abundance of grey reef sharks, there are relatively few sandbar sharks and vice versa.
A recent acoustic tagging study revealed that grey reef sharks have a relatively small home range (the part of the reef they spend most of their time in). They also studied social behaviour and found that these social groups are very stable, movement of individuals between groups rarely occurs and many of the same individuals will associate with each other year on year.
Like most other shark species, grey reef sharks mature late and reproduce slowly. They are thought to mature at around 7 years and have an estimated lifespan of at least 25 years. A viviparous species, they give birth to litters of 1-6 live pups ever other year, with gestation periods estimated at 9-14 months. When pups are born, they are usually between 45 and 60cm long.
Grey reef sharks are currently listed as endangered on the IUCN Red List, with global populations declining.
Because of their life history strategy combined with their limited home range and social behaviours, this species is especially at risk of localized depletion. They are harvested by fisheries around the world for their meat, fins and livers, but are also at risk from coral reef degradation as a result of rising sea surface temperatures and ocean acidification.
Fortunately, these sharks are protected from fishing in Egyptian waters under local laws, national and regional agreements and conventions!
This is important as they are a keystone species on the Red Sea coral reefs, managing the food web and preventing any prey species populations expanding too much, which could have a negative impact on the populations of those species at a lower trophic level (further down the food web).
RED SEA PROJECT™